Fuel cells are classified into several types according to the electrolytes or electrodes used therein. Typical types are alkaline types, phosphoric acid types, molten carbonate types, solid electrolyte types and polymer electrolyte types. In particular, polymer electrolyte fuel cells that can operate at temperatures ranging from low temperatures (about −40° C.) to about 120° C. attract attention and are progressively developed and practically used as low-pollution power sources for automobiles.
To increase the reaction rate in fuel cells and enhance the energy conversion efficiency, a layer containing a catalyst is conventionally provided on the surface of a cathode (air electrode) material or an anode (fuel electrode) material of fuel cells. (Hereinafter, electrodes in which a catalyst-containing layer is provided on the surface of a cathode material or an anode material will be also referred to as the fuel cell electrodes.)
Platinum that has high activity has been generally used as the catalyst. However, since platinum is expensive and exists in a limited amount, alternative catalysts have been desired.
Metal oxides are possible alternative catalysts to platinum. Further, to increase oxygen reduction potential, metal oxides containing nitrogen or carbon, such as metal oxynitrides, metal oxycarbides and metal oxycarbonitrides, have been studied as catalysts.
However, these metal compounds have so high electric resistance that they do not pass an electric current. Because of this drawback, electron conductive particles are used in combination therewith.
For example, Patent Literature 1 discloses an oxygen-reducing electrode material using a metal oxynitride as a catalyst.
However, even this electrode permits the passage of far below current than practically required.